Build a Shopify Profit Margin Calculator with Claude Code

Build a Shopify Profit Margin Calculator web app. People search for "shopify profit margin calculator" about 200 times a month, plus related searches like "shopify fee calculator", "ecommerce profit calculator", "shopify margin calculator". The goal is a tool that ranks for those searches and makes money.

What it should do:
Build a Shopify profit margin calculator with product cost and selling price inputs. Include all Shopify fees (transaction, payment processing, subscription), shipping costs, and taxes. Calculate gross profit, net profit, and margin percentages. Add pricing optimization suggestions and break-even analysis. Focus on getting accurate fee calculations working first.

How to build it:
1. Read this folder first. If it is empty, set up Next.js (App Router, TypeScript, Tailwind). Write a short PLAN.md with the pages, components, any library or API you need, and what you are leaving for later. Then build.
2. Put the tool itself on the homepage so a visitor can use it within seconds, with no sign-up. Do the processing in the browser where you can, so it is fast and cheap to run. Look at how Shopify Analytics, TrueProfit, BeProfit do it and make ours simpler and faster.
3. Make it rank. Use "shopify profit margin calculator" in the title tag, H1 and meta description. Under the tool, add a short how-to and an FAQ with FAQPage schema, and add a sitemap and robots.txt. Scaffold these programmatic pages from one template and a data file: /shopify-profit-calculator-[product-type], /ecommerce-margin-[business-size], /shopify-fees-[plan-type].
4. Monetisation: Freemium: 25 calculations/month, $9/mo unlimited + analytics. Add a pricing page and the free usage limit, and leave Stripe checkout as a clearly marked stub with the environment variables it needs.
5. Run it locally and test the main flow end to end, including edge cases like empty input and very large files. Fix what breaks, then finish with a summary of what works, what is stubbed and how to deploy it to Vercel.

Paste it into Claude Code in an empty folder. It plans first, so read PLAN.md and steer before it starts building.